Conservatively, there may be over one hundred thousand localities in the world waiting for the benefits that electricity can provide, and many of these locations are in climates where sunshine is plentiful. Thus, hybrid systems, such as those involving diesel engine generators, batteries and renewable energy sources, may be advantageously utilized in such locales. Cost effective applications of hybrid systems include remote facility power, remote home and village power, and power for dedicated loads, such as communications. The hybrid system market is anticipated to be one of the most important growth opportunities for photovoltaic systems and electrical power processors capable of managing power flow. However, hybrid system design is driven by economics, electrical performance goals, and predetermined needs. Most frequently, a reduction in diesel fuel consumption and engine run time is an economical necessity in countries where oil importation is a critical drain on financial resources.
Various Uninterruptible Power Supply (UPS) systems have been developed in the past. A UPS may be utilized in hybrid systems such as those involving diesel engine generators, batteries and renewable energy sources.
The use of alternative power generation systems in remote applications is becoming increasingly important. However, in developed countries, the cost of extending the power grid to remote installations may be prohibitive, while in developing countries, a local power grid may not even exist. In military or remote temporary industrial sites, such alternative power generation systems may be the only option. In these situations, the use of power generators such as diesel-fuel powered generators is often a reasonable choice. The cost of operating such a system may be reduced by supplementing the system with a battery storage system and possibly another source such as by solar or wind power. The alternate source may thus be used to assist in charging the batteries, and the batteries may be used with an inverter in place of the diesel when the batteries are charged.
The inverters used in these applications typically function under two modes of operation. In the first mode, the battery is charged; in the second mode, energy is supplied from the battery to the load instead of from the diesel. During switching between the battery and the diesel, significant transients can occur in both the voltage and the frequency supplied to the load. These transients can negatively affect sensitive modern equipment which may comprise part of the load.
Most renewable energy conversion devices such as photovoltaic cells and wind generators depend upon an unpredictable availability of the source of energy, e.g. sunshine or winds. When the devices are implemented in energy systems, they typically produce unpredictable, unregulated AC or DC power with uncontrolled frequency or voltage levels. Small systems typically collect and store energy in a battery bank, then apply DC power directly to the loads as needed, and are operated as stand-alone systems. The battery bank provides the energy reservoir for the system when the loads, the storage and the collectors are properly sized. The DC loads, such as lighting, communications, pumping, or refrigeration are readily available for low-voltage systems. For small systems with proper sizing and noncritical loads, hybridization with diesel or similar engine generator systems is unnecessary for more remote applications because the renewable source, if sized properly, can economically supply all the power needed for the system.
For larger systems requiring AC loads, the complexity of the renewable system is increased because of the addition of an inverter. The addition of an engine-generator to construct a renewable hybrid energy system adds additional complexity.
Stand-alone inverters which are not utility-interactive have been developed and evaluated in renewable energy systems. Stand-alone inverter topology and performance vary with measured efficiencies, typically ranging from 60% to 93%. The voltage waveforms of the AC outputs are typically quasi-square wave, although a small number of sine wave inverters are also available which operate as stand-alone devices having efficiencies which range from 70% to 85%.
Small UPS inverter hardware, typically 1 to 3 kW, is also currently available. Such hardware is designed to supply power to critical loads in the event of utility outages or brownouts. UPS hardware is typically designed with good power-quality goals and short duration operation, but typically do not optimize efficiency or surge capabilities. These UPS inverters and stand-alone inverters are usable in systems or hybrid systems in which either the engine-generator using non-renewable fuel or the inverter position of the system is supplying AC power to the loads.
As used herein, the term "loads" includes, but is not limited to, motors, non-linear electronic loads, highly inductive ballasts for fluorescent or vapor-arc lighting, and/or resistive loads. The load surge currents may be as high as three times the normal operating currents. Inductive kicks and switching transients of at least two times the normal peak operating voltage may be produced by the loads. Typically the power factor of the load will be less than 0.9 and the system may include lumped power factor correction, filtering, or resonant-type voltage regulators for critical or sensitive loads. Some critical loads, such as computers, may use additional UPS components.
As used herein, the term "transfer switch" includes, but is not limited to, a switch which provides timed, synchronized transfer of the electrical power flow which is transferred from the engine-generator to the inverter source and vice-versa. The transfer switch is typically a break-before-make switch that operates when the source voltages are synchronized or are at a zero crossing of the current.
As used herein, the term "inverter" refers to a device which converts DC energy stored in a battery or batteries to regulated AC voltage and current. The inverter typically may generate either a quasi-square wave (QSW) voltage or a sinusoidal wave output voltage. Typically, the QSW inverter is adequate for most non-sensitive loads, but overall system performance and efficiency are generally higher with sine wave inverters. Typically, the inverter is sized to provide system load currents continuously and must be capable of supplying starting surges, providing and sinking the Volt-Ampere Reactive (VAR) requirements of the loads, and withstanding the switching transients produced by some loads. The inverter must also be compatible with the storage medium and should tolerate normal variations in supply voltages due to changes in battery state of charge and loading. The inverter should automatically self-protect in the event of overloads for extended periods of time. Typically, transient protection of the DC input terminals is desirable.
As used herein, the term "battery" or "storage battery" is used to generally refer to a storage medium or source of DC energy. Battery types include but are not limited to deep-discharge lead-acid batteries and nickel-cadmium batteries. Those skilled in the art of battery selection will recognize which battery technology is appropriate for a specific application.
Sources of DC energy other than batteries include fuel cells, with which the present invention has been successfully applied in testing, and high speed engine generators whose variable speed precludes frequency regulation and therefore includes a multiphase rectifier, an example being a portable lightweight gas turbine. For the fuel cell, applying DC to the fuel cell assembly, previously identified as the battery charger mode of application, can be used by those skilled in electrochemistry to generate fuel for storage and future use.
The battery is typically sized to maximize the effectiveness of the renewable energy source and an engine-generator in hybrid systems. The storage battery must be capable of supplying the load surge requirements while maintaining a reasonable operating voltage at the inverter inputs. The battery type and size typically determine thresholds, charging currents and times, and control algorithms for the system. The depth-of-discharge charge rates, and thresholds of a battery typically vary with type of plates, chemistry and/or construction.
As used herein, the term "controller" may refer to, but is not limited to, a single controller, such as in a hybrid system, which may comprise multiple functions or multiple controllers. Typical functions of a controller include: monitoring and maintaining the state of charge of the batteries; monitoring and providing proper on-off cycles for the engine-generator including adequate run times and warmup times; maximizing the utilization of the renewable energy source; and providing the control signals to the transfer switch, starting circuits, inverters, and possibly to the loads when load shedding is needed. The controller may contain rectifiers for converting the generator's AC power to DC for charging the storage batteries. The controller should also contain the control algorithms needed to maximize the use of the renewable energy source, maximize the lifetime of the storage battery, and optimize the performance of the engine-generator while minimizing its maintenance.
As used herein, the term "renewable energy source" may include, but is not limited to, photovoltaics, wind devices, hydro devices such as low-head turbines, biomass generators, solar thermal systems, and/or other means of generating usable electrical power using renewable energy sources. The collector of renewable energy should supply energy to the system whenever that energy is available, except, for example, when the batteries are fully charged and the load is off. The renewable energy source should be sized and matched to the storage to maximize displacement of non-renewable fuels in a hybrid system. The amount of energy the renewable energy source can provide, the storage and the engine-generator should be also coordinated to allow the engine-generator to operate at maximum efficiency. Minimizing maintenance of the engine-generator is typically very important, as costs for service charges for repairs can represent a significant part of the cost of an ongoing system.
As used herein, the term "non-renewable energy source" may include, but is not limited to, hydrocarbon fuel burned in an engine-generator, a turbine-generator, or a thermal-electric generator (TEG). Typically, an engine-generator is chosen for its high reliability, performance and the desirability and availability of a fuel source. Diesel-power engine-generators are commonly used, although gasoline or propane generators may also be used. The engine-generator is preferably remotely controllable to allow system control for starting and stopping. The generator should be matched to the combination of load demand and the storage-battery charging requirements to maximize operating efficiency, typically at least 50 to 90% loaded.
FIG. 1 shows a simple block diagram of a renewable hybrid system using an engine-generator as a secondary electrical energy source.
FIG. 2 shows a block diagram of a standby UPS system. The utility grid is the only source of energy for this system. The utility continually charges or maintains state of charge on the battery. The battery supplies power to the loads in the event of brownouts or power failures. The inverter begins transferring the power from the battery once an out-of-spec utility condition is detected and the transfer switch is operated. The inverter is typically designed to provide good quality power to the load for a short time until backup power can be supplied, until the load can be safely turned off, or until the utility power returns.
A comparison of FIGS. 1 and 2 reveals similar topology, although some blocks may perform similar functions using different or modified algorithms.
Performance requirements and similarities between the two systems are summarized below:
1. The utility grid is the primary source of power for the UPS system. The grid is replaced by an engine-generator in the renewable energy system, which becomes the secondary source of power for the system. PA1 2. The inverter seldom transfers power in the UPS system. However, the inverter is a major power-handling element of the renewable system. Thus, the efficiency of an inverter in the UPS system is of less importance than in the renewable energy system. Furthermore, the continuous power handling capability of the inverter in the UPS can be less than the inverter for a comparably sized renewable system. PA1 3. The transfer switch performs the same function in either system, but operation is via modified algorithms and includes warm-up and minimum run-times for the engine-generator and the renewable system. PA1 4. The battery storage capacity for the UPS system is typically much smaller than the storage capacity for a comparable renewable energy-system. The charging algorithm for the batteries in a UPS system includes a constant current or taper charge immediately after a period of use, and maintaining the state-of-charge over long periods of time. The charging algorithm for the renewable system includes a high rate of charge during operation of the engine-generator with full-charge voltage signaling turn off of the engine-generator. Renewable energy sources supply energy to the batteries whenever the output exceeds the demand of the load. Full charge on the battery and extra renewable energy signal an additional regulation algorithm that may temporarily remove the renewables from the circuit or provide a battery overcharge protection algorithm to the renewable. A low battery threshold for the UPS systems signals a shutdown condition. A low battery threshold in the renewable system signals the engine-generator to start for battery recharge and continued system operation. PA1 5. The controller or controllers for either system monitor operating conditions and perform the preset algorithms as described above through appropriate signals, contact closures, or contact brakes. PA1 6. The rectifier performs the same function in both systems. The rectifier for the UPS system performs continuously and at lower current levels than the rectifier for the renewable hybrid system. The rectifiers for the renewable system operate only when the engine-generator is running.
Thus, similarities in topology and performance requirements of both the standby UPS system and the renewable system indicate that a modified UPS inverter-type system may be used in a hybrid renewable energy system. However, existing small (1-7 kW) UPS inverter hardware is neither economically nor technically feasible in a hybrid energy system. For example, necessary requirements include increased efficiency of conversion (minimum 80% with desirable at or above 90%), improved battery charging capabilities (faster charging rates when the engine-generator is operating), reduction of standby losses when the inverter is not providing power, and improved continuous power handling.
Therefore, one object of the present invention is to provide a new and novel means for providing a third mode of operation, i.e. allowing the inverter to operate in parallel with the generator.
It is another object of the present invention to allow switching from an inverter to an engine-generator without transients typically associated with such systems.
It is still another object of the present invention to provide switching by using a single power stage, wherein the same switching components are used to charge the battery, to invert the battery power in a stand-alone mode, and to invert the battery power in parallel with an engine-generator.
It is yet another object of the present invention to provide a flexible design which allows for different optimization strategies to be implemented.
It is another object of the present invention to allow an inverter to charge a battery during one part of a sine wave cycle and to deliver power from the battery during another part of the cycle during a parallel mode of operation. It is still another object of the present invention to minimize fuel consumption of an engine-generator.
It is yet another object of the present invention to decrease the time required to charge a battery.
It is still another object of the present invention to substantially eliminate harmonic distortion in load current.
It is another object of the present invention to provide a system which, in a parallel mode, adapts to arbitrary non-linear loads.
It is yet another object of the present invention to provide a power processor which can accommodate one, two, or three phase applications, wherein the processor may operate independently on each of the three phases.
It is still another object of the present invention to provide a power processor which optimizes power flow in a hybrid generator system.
Other objects of the present invention, as well as particular features, elements, and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.